A prior fuel injection system includes a common fuel rail and a plurality of fuel injectors in communication with the fuel rail for injecting fuel into a plurality of cylinders of an internal combustion engine. Each of the fuel injectors has an electronic control valve or solenoid for controlling fuel injection into a particular cylinder. An electronic control unit, or controller, is used to control the electronic control valves, as well as other aspects of the fuel injection system. The controller may include volatile and non-volatile memory, input and output driver circuitry, and a processor capable of executing one or more stored instruction sets. In operation, the controller determines an excitation or energizing duration for each control valve corresponding to current engine conditions. Energizing of a particular control valve causes the valve to open, which allows fuel injection to occur. However, imprecise determination of energizing durations may result in operating problems such as engine noise and excessive engine emissions.
A prior method for determining energizing durations by a controller involves determining a desired injection pressure and a raw injection duration, or raw pulse width, from two separate but interdependent look-up tables that each reference desired engine torque and engine speed. The raw injection duration is not based on time units, however, but rather is based on angular displacement of the engine crankshaft measured in degrees. The raw injection duration is then adjusted to establish a final injection duration based on injection pressure error, which is a function of desired injection pressure and observed or actual injection pressure. Finally, the final injection duration is converted from degrees to time to establish an energizing duration, and a corresponding control signal is sent to a particular fuel injector.
Because this method involves interdependent look-up tables for determining desired injection pressure and raw injection duration, calibration of the look-up tables and associated controller is difficult and time-consuming. Furthermore, desired injection pressure values, which are used to control a fuel pump, cannot be independently varied so as to optimally adapt injection pressure to variable operating conditions such as air temperature.
Several methods have been proposed to enhance fuel injection capabilities. One such method is known as split injection. Split injection consists of a first injection, called the pilot injection, followed by a delay, and then a second injection, referred to as the main injection. When performing split injection, precise determination of energizing durations for both the pilot injection and the main injection is essential. Many times, operating conditions at which split injection may be performed are restricted to lower engine speeds due to difficulties in establishing precise energizing durations.
Another method for determining pilot and main energizing durations is similar to the method previously described. The method involves determining a desired injection pressure based on a desired engine torque output and engine speed; determining a raw pilot injection duration, or raw pilot pulse width, based on a desired pilot engine torque output and engine speed; and determining a raw main injection duration, or raw main pulse width, based on a desired main engine torque output and engine speed. As in the above method, the raw injection durations are not based on time units, but rather are represented in degrees of rotation of the crankshaft. Furthermore, the desired injection pressure and the raw injection durations are determined in parallel from separate, but interdependent, look-up tables. The raw pilot injection duration and the raw main injection duration are then adjusted to establish a final pilot injection duration and a final main injection duration, respectively, based on injection pressure error. Next, the final pilot injection duration and the final main injection duration are converted from degrees to time to establish a pilot energizing duration and a main energizing duration, respectively, and corresponding control signals are sent to a particular fuel injector.